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BelAQI index

What is an air quality index?

To assess the air quality on a short time scale (daily/hourly), an air quality index is used. This index combines the concentration of various ambient air pollutants into one representative number. As such, technical measurement results are translated into an easily understandable assessment of the air quality.


In the context of the European "JOAQUIN"-project the Joaquin-index was developed, which has been used since one year in the Netherlands (see The BELAQI-index is a version of the Joaquin-index adapted to the Belgian situation. These "100%" health-based indexes make use of "dose-response" relationships. As starting point for the Joaquin-index, concentration scales are determined based on the short term health impact of PM2.5. This is particulate matter with an aerodynamic diameter less than 2.5 µm and is generally considered as the air pollutant with the biggest health impact.

The concentration scales of the other pollutants (nitrogen dioxide or NO2, ozone or O3, particulate matter with a diameter less than 10 µm or PM10) are determined based on comparable health impact. To do so, the “Relative Risk” (RR) as formulated in the Health Risks of Air Pollution In Europe (HRAPIE) report of the World Health Organisation (WHO) are used. A RR represents the increase in health impact caused by a concentration increase of 10 µg/m³.

The increase of air pollution affects the total daily mortality but also causes an increase in, for example, hospitalisation cases related to heart-, vascular and lung diseases. Nevertheless, to determine the impact of air pollution the RR’s related to an increase in total daily mortality are used. This is because no data for hospitalisation increase by NO2 pollution are available. As such, to include the RR’s related to hospitalisation additional assumptions are required, which would complicate the impact assessment.

The following RR’s (per 10 µg/m³ increase) from the HRAPIE report are used for increase in total daily mortality:

PM2.5 daily mean 1.0123
O3 max 8-hourly mean 1.0029
NO2 max 1-hourly mean 1.0027

This means that for each 10 µg/m³ rise in PM2.5 concentration the daily mortality undergoes a statistically significant increase of 1.23 % (likewise, for each 10 µg/m³ rise in the maximal 1-hour mean NO2 concentration the mortality increases by 0.27 %). These RR’s for total mortality are determined based on epidemiological research that compares the daily variation in air pollution with the daily mortality rate.

The starting point of the BELAQI-index scale is PM2.5 and the same methodology is used as was the case with the Joaquin-index. Based on the guideline values and interim targets of the WHO for PM2.5, four “health”-concentration classes are made:

- Class A with daily mean PM2.5 concentrations smaller than the yearly mean WHO guideline value: <= 10 µg/m³
- Class B with daily mean PM2.5 concentrations between the yearly and daily mean WHO guideline values: > 10 µg/m³ - <= 25 µg/m³
- Class C with daily mean PM2.5 concentrations between the daily mean WHO guideline value and the WHO interim target II: > 25 µg/m³ - <= 50 µg/m³
- Class D with daily mean PM2.5 concentrations larger than the WHO interim target II: > 50 µg/m³.

In order to work with 10 index classes these four classes are further subdivided, as shown for the BELAQI-index in Figure 1:

1 0 - 5
2 6 - 10
3 11 - 15
4 16 - 25
5 26 - 35
6 36 - 40
7 41 - 50
8 51 - 60
9 61 - 70
10 >70

Figure 1: PM2.5 classification according to the WHO values (blue: class A, green: class B, orange: class C, red: class D).

The classification (Figure 1) is slightly different from the classification used to determine the Joaquin-index.

Table 1 presents the index-concentration scale and the corresponding increase in daily mortality based on the RR for PM2.5:

Index PM2.5
daily mean
Increase in daily mortality
compared to a concentration
of 0 µg/m³ (%)
1 0 0
2 5 0.615
3 10 1.23
4 15 1.845
5 25 3.075
6 35 4.305
7 40 4.92
8 50 6.15
9 60 7.38
10 70 8.61

Table 1: BelAQI index scale, associated daily mean PM2.5 concentrations and corresponding increase in daily mortality.

Using the RR’s for NO2 and O3 from the HRAPIE report, the concentration scales that cause an equal increase in mortality are calculated, resulting in the index-concentration scales presented in Table 2:

Index O3
max 8-hourly mean per day
max 1-hourly mean per day
1 0 0
2 21 23
3 42 46
4 64 68
5 106 114
6 148 159
7 170 182
8 212 228
9 254 273
10 297 319

Table 2: BelAQI index scale and associated maximal 8-hour O3 and 1-hour NO2 mean concentrations based on an equal increase in mortality compared to the daily mean PM2.5 concentrations.

With respect to O3, the relation between concentration and health effects is determined based on the “maximal 8-hour mean” concentration of a given day. As European information and alert thresholds are formulated on a 1-hourly scale, it was decided to use hourly mean concentrations for the O3 sub-index scale. This is scientifically sound due to the strong correlation between the maximal 8-hour and the maximal of a given day (R² =0,97 for the daily maxima of a “virtual” or “mean” Belgian O3 station during the period 2013 – 2015). A maximal 1-hour mean O3 concentration corresponds to 1.11 times the maximal 8-hour mean O3 concentration (1.11 being the median).

With respect to PM10, no RR for daily mortality is defined in the HRAPIE report. However, it was decided to also develop a PM10 sub-index scale, as the information threshold for particulate matter (50 µg/m³, implemented the 1st of November 2016) as well as the smog alert threshold (70 µg/m³, in use since 2006) are based on PM10 concentrations. To determine a (sub-)index scale for PM10, the PM2.5/PM10-ratio is used. This ratio amounts to 0.63 and is the median of the daily mean PM2.5/PM10-concentrations of a “virtual” or “mean” Belgian O3 station during the period 2013 – 2015. Using this ratio, the PM10 concentration scales can be determined based on the already existing PM2.5 scales.

In Table 3, the concentration scales for PM10 (calculated via the PM2.5/PM10-ratio), PM2.5, O3 (recalculated from maximal 8-hour to maximal 1-hour mean concentrations) and NO2 are given.

Index Classification PM10 daily mean
PM2.5 daily mean
O3 max 1-hourly mean per day
NO2 max 1-hourly mean per day
1 Excellent 0 0 0 0
2 Very good 8 5 24 23
3 Good 16 10 47 46
4 Fairly good 24 15 71 68
5 Moderate 40 25 118 114
6 Poor 56 35 165 159
7 Very poor 63 40 188 182
8 Bad 79 50 235 228
9 Very bad 95 60 282 273
10 Horrible 111 70 330 319

Table 3: BelAQI index scale with PM2.5 daily mean concentrations and corresponding PM10 daily mean (based on the PM2.5/PM10-ratio), O3 maximal 1-hour mean (recalculated from maximal 8-hour mean) and NO2 maximal 1-hour mean concentrations (based on equal increase in mortality compared to PM2.5 daily mean concentrations).

In order to develop a health impact classification, the concentration scales (Table 3) are rounded while taking into account the NO2 threshold of 200 µg/m³, the NO2 phase 1 threshold of 150 µg/m³ in Brussels, the European O3 information and alert thresholds of 180 and 240 µg/m³ respectively, the PM10 information threshold of 50 µg/m³ in Belgium, the smog alert threshold of 70 µg/m³ and the phase II threshold of 100 µg/m³ of the regional smog action plan of Brussels. The result is shown in Table 4.

Index Classification PM10 daily mean
PM2.5 daily mean
O3 max 1-hourly mean per day
NO2 max 1-hourly mean per day
1 Excellent 0 - 10 0 - 5 0 - 25 0 - 20
2 Very good 11 - 20 6 - 10 26 - 50 21 - 50
3 Good 21 - 30 11 - 15 51 - 70 51 - 70
4 Fairly good 31 - 40 16 - 25 71 - 120 71 - 120
5 Moderate 41 - 50 26 - 35 121 - 160 121 -150
6 Poor 51 - 60 36 - 40 161 - 180 151 - 180
7 Very poor 61 - 70 41 - 50 181 - 240 181 - 200
8 Bad 71 - 80 51- 60 241 - 280 201 - 250
9 Very bad 81 - 100 61 - 70 281 - 320 251 - 300
10 Horrible >100 >70 >320 >300

Table 4: BelAQI index scale, health impact classification and associated concentration scales for PM10 daily mean, PM2.5 daily mean and O3 and NO2 daily maximal 1-hour mean.

Remark: to calculate the actual (hour per hour varying) sub-indexes and the global index, the concentration scales of Table 4 are applied to the latest hourly mean O3 and NO2 concentrations and the running 24-hourly mean PM2.5 and PM10 concentrations.

Global air quality index and classification

To determine the global air quality index, the sub-index for each pollutant is calculated based on the index scale in Table 4. The highest sub-index defines the global air quality index.

10 qualifications are used, going from “excellent” to “horrible”. The classification of the index is established in function of the severity of the health impact (mortality risk). Meanwhile, the information and alert thresholds for PM10 and O3 are taken into account. If the particulate matter (PM10) concentrations exceed the smog information threshold, the air quality is qualified as “poor” (6). In the case of an exceedance of the PM10 alert threshold, the index level amounts to eight and the air quality is described as “bad”. With respect to O3, the (sub-)index in case of European information threshold exceedance is one class higher compared to PM10, resulting in the air quality label “very poor” (7). If the O3 alert threshold is exceeded, the air quality is qualified as “bad” (8).

The BELAQI-index gives no indication of the long term air quality. The index is meant to assess short term effects of the daily variation of air pollution. It is also important to note that an index evaluates the air quality in a qualitative way (and thus not quantitatively). Consequently, it is not recommended to use trends or statistics of air quality based on an air quality index. This is also the reason that the calculated indexes are not used in the yearly air quality reports.